Host response to the invasion of tissues by the pathogenic yeast, Candida albicans, is variable but with oral thrush and eosphagitis as seen in AIDS patients. The accompanying pseudomembranous lesion is composed of white cells along with fungal elements, bacteria, edema, and microabscesses. A number of host factors contribute to inflammation in such lesions. Certainly, complement, as an opsonin in phagocytosis and as an attractant for white cells contributes significantly to the host response. However, in the pathogenesis of infections such as oral candidiasis, many of the reactions in which complement participates are only descriptive. In this application, a molecular characterization of Candida-complement interactions will be pursued. We will purify, clone and determine the functional significance of the C3d receptor of Candida albicans. C3d is a cleavage product of C3b, a complement intermediate formed by either the classical or alternative pathway C3 convertase. C3d as well as other fragments of C3 are recognized by surface receptors of mammalian cells which mediate phagocytosis and/or are involved in the control of the immune response. Thus, these receptors (and their C3d by products) play an important role in pathogenesis. In previous work, I have identified the C3d receptor of C. albicans and found it to be preferentially expressed on hyphae but not yeast forms. The receptor was partially purified and a monoclonal antibody was developed which appeared to be specific for the Candida receptor. Utilizing this monoclonal, we will (1) purify the receptor from C. albicans hyphae and characterize its oligosaccharide and amino acid composition and sequence; (2) clone the receptor protein. We will isolate the gene which encodes for the receptor protein and characterize its structure; and (3) isolate mutants of C. albicans which lack receptor activity, utilizing enrichment procedures such as resistance to lectins or tunicamycin or non-reactivity to the monoclonal antibody which recognizes the receptor protein. The mutants will be evaluated for changes in functional activities such as virulence, adherence to mammalian cells in vitro, phagocytosis and the ability to induce cell-mediated immune mechanisms. In completing these studies, we will be able to determine the relationship of this receptor to pathogenesis. Aside from the characterization and determination of the importance of the receptor in pathogenesis, the information obtained will be useful in a comparative way since mammalian cells have similar receptors.